Optimization of flood risk reduction through multiple lines of defence

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1 Delft University of Technology Optimization of flood risk reduction through multiple lines of defence van Berchum, Erik; Jonkman, Sebastiaan N.; Timmermans, Jos; Brody, S.D. Publication date 2017 Document Version Publisher's PDF, also known as Version of record Citation (APA) van Berchum, E., Jonkman, B., Timmermans, J., & Brody, S. D. (2017). Optimization of flood risk reduction through multiple lines of defence Abstract from 7th International Conference on Flood Management 2017, Leeds, United Kingdom. Important note To cite this publication, please use the final published version (if applicable). Please check the document version above. Copyright Other than for strictly personal use, it is not permitted to download, forward or distribute the text or part of it, without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license such as Creative Commons. Takedown policy Please contact us and provide details if you believe this document breaches copyrights. We will remove access to the work immediately and investigate your claim. This work is downloaded from Delft University of Technology. For technical reasons the number of authors shown on this cover page is limited to a maximum of 10.

2 Optimization of flood risk reduction through multiple lines of defence Berchum, E.C. van* 1, Jonkman, S.N.*, Timmermans, J.S.*, Brody, S.D.** * Delft University of Technology, Delft, the Netherlands ** Texas A&M University, College Station, TX, USA 1 Delft University of Technology, Faculty of Civil Engineering and Geosciences, Stevinweg 1, 2628 CN Delft, the Netherlands KEYWORDS: Flood defences, flood risk reduction, economic optimization, flood risk modelling INTRODUCTION Floods can have a huge impact on the regions they affect. The impact of these disasters can be reduced with the use of flood risk reduction measures. Flood-prone regions often require a combination of interventions to reduce the risk to an acceptable level. The amount of risk reduction provided by various interventions can be quantified using probabilistic risk analysis. The interdependence between multiple lines of flood defences within the same system can have a large effect on the region. For example, the height and strength of a coastal levee greatly affects the impact of any inland flood risk reduction measures, like vegetation for wave attenuation. This can be investigated with the use of probabilistic risk assessment. However, assessing such flood defence systems can be computationally very intensive, as well as timeconsuming. Although methods have been developed to optimize a single type of intervention (e.g. defences (Kind, 2014; Duipuits and Schweckendiek, 2015)), there are no generic approaches that address combinations of interventions. In this paper, a new model is presented that is able to (1) simulate combinations of flood risk reduction measures and (2) optimize these based on costs, economic risk reduction, and environmental impact. A key feature of the model is the capability to incorporate different types of interventions, including barriers, dikes, wetlands, non-structural interventions, modifications of structures, and buyouts. This is kept computationally workable by using simple, yet realistic representations of the system elements in the form of fragility curves, cost curves, and damage curves. Other aspects such as societal and environmental impacts will be discussed qualitatively and ranked with other indicators.

3 SIMULATION MODEL The model is built up out of three parts: the Damage Model, the Risk calculation and the Optimization Model. Figure 1 shows how the different parts interact and which actions are included. Figure 1 - Schematization of the interaction between the Damage Model, the Risk Calculation which cycles through the Hydraulic Boundary conditions and the Optimization Model, which cycles through the Flood risk reduction strategies. The Damage Model calculates the construction cost, environmental impact and estimated damage cost for a single storm. This is done by combining three layers of information for the region: the Region lay-out, the Flood risk reduction strategy consisting of the chosen combination of flood risk reduction measures and the Hydraulic boundary conditions. This is illustrated in Figure 2. The impact of a storm on the region and the flood risk reduction strategy is calculated using simplified hydraulic formulas (Jonkman & Schweckendiek, 2015; Van der Meer et al., 2016). Figure 2 The three layers of a region as depicted in the model

4 The Risk calculation uses this combination to calculate the estimated value of damage. It repeats the calculations from the Damage Model for storms with different return periods. With this information, it is possible to construct a risk curve. The basis is a set of (flood) scenarios with their probablities and consequences (cf. Kaplan and Garrick, 1981) The graph shows the probability of exceedance of an event with a certain damage level. The expected damage can also be computed from this information. How this risk curve is constructed by combining the Damage Model with the Risk calculation, can be seen in figure 2. Figure 3 graphical representation of how the Risk curve (the dotted line) is constructed by combining the results of different damage calculations as part of the Risk calculation. The rectangles illustrate how the total expected value of damage is derived by numerically integrating the risk curve. EVALUATION TOOL The strength of the model is the ability to compare large numbers of strategies with both structural and non-structural flood risk reduction measures, such as levees, oyster reefs, improving evacuation routes, and steering urban development locations. This evaluation is done in the Optimization Model. It provides the input for the Damage Model, analyzes the output, and investigates how the risk profile of the region reacts to different design choices, for example by identifying design trade-offs. By searching for design trade-offs, one could think of trade-offs between project goals (e.g. High environmental scores are hard to achieve in combination with low barrier heights inland ) or trade-offs between design choices (e.g. Placing a relatively low barrier at the coast significantly diminishes the impact of Nature-based solutions in the first protected area ). SYSTEM OPTIMIZATION A stylised case study based on the Houston-Galveston Bay Area in Texas was used to demonstrate the model (van Berchum, 2017). It was able to show how the region reacts to design choices, for example by providing insight into the change of effectiveness of

5 wetlands around the bay depending on the investments in coastal structures. The case study included both structural and non-structural measures, ranging from levees and storm surge barriers to Nature-based Solutions like wetlands, and spatial planning measures like raising insurance premiums. Figure 3 map of the Galveston Bay Area with implemented and identified possible flood risk reduction measures. The model is designed to be used as a decision-making tool during early phases of a design. Especially during the conceptual design phase, when design choices are impactful and information is scarce, it can provide valuable insights and save precious time and money in finding attractive solutions for reducing risks for high-vulnerability areas threatened by coastal flooding. REFERENCES Kind, J.M. (2014). Economically efficient flood protection standards for the Netherlands. Journal of Flood Risk Management 7(2): Zwaneveld, P. and Verweij, G. (2014). Economisch optimale waterveiligheid in het IJsselmeergebied. Technical Report 10, CPB, The Hague, 2014 Dupuits, E.J.C. and T. Schweckendiek (2015). Flood Risk and Economically Optimal

6 Safety Targets for Coastal Flood Defense Systems. Proceedings of the 12th International Conference on Applications of Statistics and Probability in Civil Engineering (ICASP12), Vancouver, Canada, July Kaplan, S., & Garrick, B. J. (1981). On the quantitative definition of risk. Risk analysis, 1(1), Berchum, E.C. van (2017). Flood Risk Reduction System Optimization. Application to the Galveston Bay Area. Interim Report. TU Delft, Delft. The Netherlands Van der Meer, J. W., Allsop, N. W. H., Bruce, T., De Rouck, J., Kortenhaus, A., Pullen, T.,... Zanuttigh, B. (2016). Manual on wave overtopping of sea defences and related structures. An overtopping manual largely based on European resreach, but for worldwide application. Retrieved from Jonkman, S. N., & Schweckendiek, T. (2015). Flood Defences. Lecture Notes CIE5314. Delft: Delft University of Technology.